In vitro systems employing tumor cells in culture are widely used to study mechanisms of anticancer drug action and to screen for new anticancer drugs. Compounds which require metabolic activation by the liver lack activity in such systems. This limits the scope of in vitro tumor cell assays for detecting antitumor activity and its ability to predict drug sensitivity in vivo. Although we have previously reported the utility of exposing tumor cells to cofactor-fortified hepatic microsomes prior to culture and colony formation in soft-agar, not all prodrugs are activated by cytochrome P-450. In addition, hepatic microsomes cannot be incorporated in the soft-agar colony formation assay because they inhibit cell growth; thus their use is limited to relatively short-term pre-incubations. Our preliminary experiments suggest that rat hepatocytes are a much more efficient drug activating system for the soft-agar colony formation assay than hepatic microsomes. Hepatocytes offer a far wider range of drug metabolizing enzymes than hepatic microsomes as well as providing cellular barriers to diffusion and transport of reactive metabolites similar to those in vivo. We have also found that hepatocytes are capable of stimulating the growth of tumor cell lines in soft-agar culture. Studies we propose are first, to define the optimum conditions for activation of a number of anticancer drugs by hepatocytes in conjunction with the soft-agar colony formation assay. Second, we will define optimum conditions for stimulation of cell growth of human tumor cell lines and primary human tumors. Third, we will try to develop methods for hepatocyte cryopreservation which would enable us to use human hepatocytes. Finally, we will study some of the properties of the growth stimulatory factor(s) released by hepatocytes.